For that, Kisirani has an answer: because Blizzard doesn't want it to be. I have to say, I'm impressed so far with how Blizzard has tied these new class and race combinations into the lore of the world. We've only seen a little bit of how they'll do it, but while these new class/race combos could have easily just fallen into fan service (and that's definitely what I thought they were when Blizzard introduced them -- they'd abandoned all semblance of lore just to give players more options to play), it definitely doesn't seem that's the case. According to Kisirani, Blizzard is still thinking seriously about what races and classes fit together, and presumably all of the new combinations (yes, even Human Hunters, can't wait to see that one) will be explained in the end.

Good to hear. Of course, the reason they're including these combinations is partly fan service, and it's likely also meant to help folks rerolling to see all the new Cataclysm content. But knowing that there'll be lore behind all of the new combos is pretty darn exciting.